1. Field of the Invention
The present invention relates to the field of counters and, more specifically, to monotonous counters with an irreversible count variation in a single direction.
2. Discussion of the Related Art
Currently, to form monotonous up-counters, fuse elements have to be used, which have the major disadvantage of causing a destructive programming often incompatible with a programming during operation of the integrated circuit containing the fuse element. Another example relates to EPROM or EEPROM memories, the manufacturing of which requires steps not directly compatible with a CMOS technology.
Further, in both cases, the achieved programming state is either erasable (case of EEPROMs) or optically visible, which is prejudicial in security-type applications for which an inviolable count is desired to be available.
An example of application consists of counting the number of events with respect to a limiting event threshold (number of uses of a program or duration of use). In this type of application, it is generally desired to have a counter, each bit of which respectively takes states 0 or 1 in irreversible fashion to provide a binary word representative of the number of events having occurred. For example, the number of accesses to a flash-type memory may be counted. In this application, to have this counting function performed by the actual flash memory, a very large surface area of the memory must be monopolized since the “granularity”, that is, the minimum number of cells which must be addressed for an irreversible programming, is significant. In practice, to use as a one-time programming memory a region of a flash memory, a capacity on the order of one kilobit must be reserved to obtain eight one-time programming memory bits.
The other one-time programming memory cell techniques which would be likely to be used in monotonous counting operations are in practice inexploitable. For example, cells of fuse or anti-fuse type require programmings by destructive currents which are often incompatible with the product being manufactured.